Terminal alkynes contain spC−H group that is acidic in nature. Acidity also depends on the stability of the conjugate base formed.
−NO2 is electron withdrawing group and therefore decreases the electron density on the carbanion and thus stabilises the conjugate base. −NH2 is electron donating group and therefore increases the electron density on the carbanion and thus destabilises the conjugate base. Therefore, the acidity order is II>I>III.